Two designs of heat exchangers are presently in general use as reboilers and condensers in air separation and similar cryogenic applications.
The most common of these exchangers is the plate-fin brazed aluminum heat exchanger. In a typical heat exchanger: aluminum plates from about 0.03 to about 0.05 inches thick are separated by a corrugated aluminum sheet which serves to form a series of fins perpendicular to the plates and channels or flow paths arranged axially in the direction of the flow. Usually, the fin or corrugated sheets have a thickness of about 0.008 to about 0.012 inches with about 15 to about 25 fins per inch and a fin height of about 0.2 to about 0.3 inches. A heat exchanger is formed by brazing an assembly of alternate layers of flat plate and corrugated sheet with the first and last layer generally being a flat plate, with the edges enclosed by side bars.
This type exchanger is immersed in a bath of the liquid to be boiled with the parting sheet and fins oriented vertically. The boiling liquid enters a heat transfer passage (defined by the corrugation closed by a plate) through the open bottom of the heat exchange passage and the heated mixture of liquid and vapor exit from the open top of the passage. Alternative passages separated by the parting sheets contain the boiling and condensing fluids. In the condensing passage, vapor is introduced at the top through a manifold welded to the side of the exchanger and having an opening into alternate passages, defined by a corrugated sheet and adjacent plate. In each condensing passage, the vapor is distributed by a special arrangement of distributor fins. The condensate leaving the passages passes through an arrangement similar to the entering vapors. The disadvantages of this type of heat exchanger are the inability of the liquid and the vapor flows to easily redistribute in a direction perpendicular to their flow, the very close spacing of the fins in order to obtain a large secondary heat transfer surface, and the relatively large spacing between the parting sheets which limits the primary surface area for heat transfer.
A second type of heat exchanger, in current use, is the vertical shell and tube reboiler. To achieve a sufficiently low temperature difference in this design, the tube surfaces are coated with a porous enhanced boiling surface. The disadvantage of the shell and tube design is the limited heat transfer surface that can be accommodated in the distillation column.
A third heat exchanger construction is described in U.S. Pat. No. 3,840,070. This design utilizes a corrugated plate format in which plates of dissimilar geometry are stacked in pairs to form adjacent heat transfer channels for boiling and condensation. The design is asymmetric and would not generally be considered acceptable for oxygen boiling service because of the possibility of promoting dry boiling.